Possible Flaw Identified in I-35W Bridge Design

Author / Coordinator: Tony Kennedy, Mike Kaszuba, Paul McEnroe and Dan Browning
Star Tribune
August 2007

Federal officials investigating the Interstate 35W bridge disaster said Wednesday that they are looking at a possible design flaw in some of the steel plates under the bridge and issued an alert that added weight from construction work may have been a factor in its collapse.

Opening a new window into last week’s fatal bridge collapse, the National Transportation Safety Board (NTSB) said that one of its areas of inquiry involves the design of steel connecting plates known as gusset plates; the material makeup of those plates; and the loads and stresses they bore.

Hours later, Secretary of Transportation Mary Peters said the NTSB indicated that the stress on the bridge’s gusset plates may have been a factor in the bridge collapse and that one possible stress may have been the weight of construction equipment and materials on the bridge.

Peters issued the first national alert to stem from the disaster, telling bridge engineers nationwide to "carefully consider the additional weight placed on bridges during construction or repair projects." An NTSB official stressed again that the probe is in its early stages and that the design of the gusset plates is just one of many areas of inquiry.

When the I-35W bridge fell during evening rush hour last Wednesday, Progressive Contractors Inc. (PCI) of St. Michael, Minn., was at work on a $9 million job to make surface repairs. Crews from the well-known contractor had been on the bridge for six weeks.

Piles of sand, gravel were on bridge

An aerial photo taken about three hours before the accident showed several large company trucks, along with sand and gravel piles, in the two closed southbound lanes just south of the center of the bridge. The company has insisted the repairs did not cause the bridge’s failure.

The photo also shows traffic in each direction confined to two lanes, with the remaining two lanes in each direction closed for construction. At the time the photo was taken, a string of widely spaced traffic can be seen moving in each direction.

At Wednesday’s MnDOT news conference, construction engineer Liz Benjamin confirmed that workers had dumped large sand and gravel piles on top of the bridge the day of the collapse.

As had been routinely done on previous days, those piles were put on the bridge before 2 p.m., according to a source familiar with the construction project. Those piles were big enough to make concrete for a section equal to a quarter of the length of the bridge and a quarter of its width, the source said. Even if the piles weighed 100 tons, the source said, it would be lighter than the weight of three loaded semitrailer trucks.

"We continue to believe that our concrete repair work was routine, that it was done well, and that there was nothing about it that should have caused a bridge to collapse," Tom Sloan, a vice president for PCI, said in his first comments since the collapse.

He said the company had interviewed every employee on the bridge when it collapsed and had not found anyone who could substantiate reports from a first responder who said workers told him the bridge was wobbling before it fell.

The steel truss underside of the 40-year-old bridge was built with hundreds of gusset plates — metal plates of various sizes where vertical, horizontal or diagonal beams are tied together. The NTSB said Wednesday that its investigators "observed a design issue" with the plates but wouldn’t specify the location or explain the potential flaw.

The source familiar with the construction project said investigators are questioning whether the plates should have been thicker in the original design.

State transportation officials shed no light on the topic. From the outset, all parties to the investigation have noted that the failure of one connection or member in the bridge’s steel superstructure could have caused it to fall because the bridge was built without redundancy.

Plates prone to fatigue

The New York Times reported that a consultant hired by the state after the collapse to investigate what had gone wrong first discovered the potential flaw. However, a MnDOT spokesman said he had no knowledge of that being accurate and that nobody from the consulting firm had told MnDOT of such a finding.